MBA with Syslinux on USB

I'm trying to boot a Linux kernel built with Buildroot from USB on my Yosemite Macbook Air. I have followed the first steps in Failed To Install syslinux on USBDRIVE with Mac OS X, i.e. - DiskUtility stuff - Syslinux stuff

At this stage I wanted to verify that the boot loader works, so I placed a simple one liner syslinux.cfg file on the USB:

SAY Hello

When I boot from the USB, by holding the 'alt' key, I expect a "Hello" on the screen, but nothing happens. It just hangs with semi-dark screen — not completely dark. Any suggestions?